Inferior Vena Cava Filters and Complications: A Systematic Review

Inferior vena cava (IVC) filters have been used since the 1960s to treat patients with acute risk of pulmonary embolism (PE) to prevent migration of thrombus by trapping it within the filter. Traditional usage has been in patients with contraindication to anticoagulation that carry a significant mortality risk. In this systematic review, we sought to evaluate complications associated with placement of inferior vena cava filters based on published data from the past 20 years. A search was performed on October 6th, 2022, in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for systematic reviews, using three databases (ProQuest, PubMed and ScienceDirect) for articles published between the dates of February 1, 2002 and October 1, 2022. Results were filtered to include full-text, clinical studies, and randomized trials written in English pertaining to keywords “IVC filter AND complications”, “Inferior Vena Cava Filter AND complications”, “IVC filter AND thrombosis” and “Inferior Vena Cava Filter AND thrombosis”. Articles identified by the three databases were pooled and further screened for relevance based on inclusion and exclusion criteria. Initial search results yielded 33,265 hits from all three databases combined. Screening criteria were applied, with 7721 results remaining. After further manual screening, including removal of duplicate hits, a total of 117 articles were selected for review. While there are no consensus guidelines for best practice, there is compelling evidence that IVC filters can provide significant protection against PE with minimal complications if the treatment window is appropriate. Increase in the variety of filter models has led to broader availability, but skepticism remains about their efficacy and safety, with ongoing controversy surrounding appropriate indications. Further research is needed to establish clear guidelines on appropriate indications for IVC placement and to determine time course of complications versus benefits for indwelling filters.


Introduction And Background
Inferior vena cava (IVC) filters are implantable devices placed by vascular surgeons or interventional radiologists into the inferior vena cava to prevent the migration of a thrombus to the pulmonary vasculature [1]. Filters are typically inserted through the femoral, jugular, or antecubital veins, with the infrarenal IVC as the primary target of placement. Placement is performed with imaging assistance to ensure proper guidance. There are two general types of filters -permanent and retrievable. The scope of indications for IVC filter placement, however, remains controversial because of lack of definitive evidence supporting mortality benefit beyond the traditional indication of a patient with significant pulmonary embolism (PE) risk who is contraindicated for anticoagulation. IVC filter use has also been expanded due to the availability of retrievable filters without any definitive changes in guidelines for their use [2]. In accordance with the Society of Interventional Radiology (SIR), indications for placement include: (1) for patients with documented venous thromboembolism (VTE) and classic indications including: contraindications to anticoagulation, complication of anticoagulation requiring discontinuation, failure of anticoagulation, and worsening of deep vein thrombosis (DVT) during anticoagulation treatment; (2) for patients with documented VTE and expanded indications including: iliocaval or large free-floating proximal DVT, inability to maintain sufficient anticoagulation, massive PE with remaining DVT and recurrent risk for subsequent PE, recurrent PE with IVC filter placement, VTE with limited cardiopulmonary reserve, patients at high risk of complications from anticoagulation, and chronic VTE treat with thromboendarterectomy; and (3) for primary prophylaxis in patients without VTE. The most common indication is for patients with a history of VTE that have a contraindication to anticoagulants [2][3][4][5]. Specific indications for IVC filters are variable depending on organizations and their guidelines. According to the European Society of Cardiology, IVC filters are not recommended for prophylactic placement, for free-floating thrombus, or prior to systemic thrombolysis, surgical embolectomy, or pulmonary thromboendarterectomy. The only agreed-upon indication appears to be patients who have a past history of VTE, are at high risk of PE, and who have a contraindication to anticoagulation treatments [6].
Permanent IVC filters, meant to be left in the patient indefinitely, were first established for use in patients with VTE in 1967 with the introduction of the Mobin-Uddin group filter. Permanent filters offer no option of thrombophilia, relates to the constitution of blood that increases risk of thrombus formation. Various clinical and disease statuses are associated with hypercoagulability such as pregnancy, use of oral contraceptives, protein C deficiency, protein S deficiency, homocystinuria, and others. Stasis refers to alterations that typically decrease velocity of blood flow. It is believed that slowed blood flow reduces exposure to cell proteins that trigger a natural anticoagulation pathway, resulting in thrombus formation. Stasis can be seen in several situations such as atrial fibrillation, prolonged immobility, extensive surgery, or long travel. Injury to the vascular wall can cause alterations in normal blood flow. Flow disturbances create disordered currents that increase friction of flow through a vessel. Endothelial damage results from a variety of factors: smoking, atherosclerotic disease, chronic hypertension, inflammation, medical devices, etc. [14]. IVC thrombosis has commonly been associated with IVC filters. Following Virchow's Triad, IVC filters can affect both stasis and the endothelium, resulting in a possible increased risk of thrombus formation.

Methods of Filter Placement
Prior to 1989, all IVC filters were placed by vascular surgeons. Complications can arise during the placement of the filter itself. Bleeding and acute thrombosis at the surgical site are most common. Immediately after placement, filter tilt, which is defined as angulation greater than 15 degrees from the longitudinal axis of the IVC, and filter migration, change in position more than 2 cm, can also occur [15]. Human error can also be a cause of procedure complications such as misplacement or disorientation. The quantity of IVC filters utilized increased dramatically when the percutaneous delivery method was introduced, allowing interventional radiologists to predominate as the primary clinicians involved in filter placement. Lin et al. retrospectively analyzed 592 patients who underwent filter placement between March 1987 and December 2000. Complications such as insertion site hematoma or DVT, filter migration, and IVC thrombosis were compared between those who had operative IVC filter placement by vascular surgeons and percutaneous delivery by interventional radiologists. Complication rates between the groups were not statistically different (P=0. 48). No mortality was directly related to filter placement. Chronological analysis revealed that while radiologists rocketed to become the primary clinicians in filter placement after 1989, introduction of an endovascular program for vascular surgeons in 1994 resulted in a small resurgence of filter placement by surgeons with continuous steady increases. Based on the similar complication rates, Lin et al. suggest that a multidisciplinary team of radiologists and surgeons would provide optimal patient care in IVC filter placement [16].
Typically, IVC filters are placed guided by fluoroscopy in dedicated angiographic suites. Ganguli et al. sought to compare the safety and associated complications of placement via ultrasound at bedside versus traditional fluoroscopy. Between 2009 and 2011, 117 patients received IVC filters at bedside in the ICU and 571 patients underwent fluoroscopy-guided placement. Complications related to placement occurred in 4.3% of patients who received bedside filters. These complications included four cases of malpositioning and one severe tilt. Fluoroscopy generated one malposition and one severe tilt resulting in a complication rate of 0.6% (P=0.01). Complications related to indwelling time occurred similarly between both groups. Median indwelling time to complication was 74 days for the ultrasound cohort and 127 days for the fluoroscopy group. In the ultrasound group, DVTs occurred in 13.7%, PE in 5.1%, and filter thrombosis in 3.4% of the cohort. Comparatively, the fluoroscopy group resulted with DVTs in 13.3%, PE in 4.0%, and filter thrombosis in 3.9% (P=0.92, P=0.61, P=0.82, respectively). Ultimately, this study concluded that placement at bedside guided by ultrasound is a safe procedure but is associated with more complications than fluoroscopy [17].
There is difficulty in placement of filters in patients who have suffered head trauma and require intracranial pressure monitors as these patients are unable to lay supine for traditional fluoroscopy or beside ultrasound. Searching for a safe alternative, Joseph and Lopera performed a retrospective review in 2021 analyzing digital radiograph (DR) guided filter placements at ICU bedside in patients with increased intracranial pressure (ICP). A total of nine filters were placed guided by DR (eight with indication for prophylaxis, one for acute DVT). Filters placed included four Denali (Becton Dickinson, Tempe, AZ, USA), three Option Elite (Argon Medical Devices, Plano, TX, USA), and two Celect (Cook Medical, Bloomington, IN, USA). Two deaths occurred that were not related to the procedure. All nine filters were placed successfully at the level of the lowest renal vein. Average pre-procedure ICP was 16 mmHg, procedural was 13 mmHg, and post-procedure ICP was 16 mmHg. Thus, there was no significant difference in pre and post-procedural ICP (P=0.77). Filter tilt was reported as a complication in one of the Option Elite filters. Joseph and Lopera found that DR is a safe alternative method of IVC filter placement, primarily indicated in patients with increased intracranial pressure who are unable to lay supine [18]. Walker et al. examined a cohort of 129 patients, 48 who received filters guided by DR at bedside and 81 who underwent traditional fluoroscopy. Filter positioning and tilt were evaluated with post-procedural cavograms and Computerized Tomographs (CTs). Both groups experienced 100% technical success in placement. Bedside placement did have a significantly longer procedure duration compared to fluoroscopy: 14.5+/-10.2 versus 6.7+/-6.0 min (P<0.0001). However, DR at bedside was associated with significantly reduced radiation exposure: 25 mGy  vs 256.94 mGy +/-158. 6. Common complications did occur in both groups but were not significantly different: malpositioning (P=0.31), degree of filter tilt (P=0. 33), and other complications (P=0. 65). The authors concluded that there was no significant difference in outcomes when comparing bedside DR to traditional fluoroscopy placement [19].
A common complication that tends to arise with IVC filters is mechanical tilt of the filter during or after insertion. Xiao et al. sought to determine if utilizing the introducer curving technique could aid in reducing filter tilt during transfemoral insertion of Gunther Tulip filters (Cook Medical). One hundred eight patients receiving IVC filters were randomly divided into Group C and Group T, with Group T adopting the introducer curving technique. The post-implantation filter tilting angle and adherence of retrieval hook to the vascular wall was measured and compared between groups. Average post-implantation filter tilting angle in Group C was found to be 7.1±4.52 degrees, and in Group T was 4.4±3.20 degrees. Thus, the study found a statistically significant difference in average post-implantation filter tilting angle between groups (t=3.573, P=0.001). There was also a statistically significant difference in post-implantation filter tilting angle between right and left approaches (5.1±3.82 vs. 7.1±4.59, t=2.301, P=0.023). Additionally, Group T displayed a significantly lower rate of severe tilt (post-implantation filter tilting angle ≥ 10o) than Group C (9.3% vs. 24.1%, χ2=4.267, P=0.039). Retrieval hook adherence was also found to be statistically significant between Group T and Group C (2.9% vs. 24.2%, χ2=5.030, P=0.025, respectively). Ultimately, Xiao et al. found that the introducer curved technique of insertion was effective in reducing filter tilt during transfemoral insertion of the Gunther Tulip filter [20].
Choi et al. analyzed complications associated with Denali filter placement at different venous access sites. Three access points were compared: right femoral, left femoral, and right jugular. Variables measured included IVC diameter, degree of filter tilt, filter tip abutment/limb penetration, fluoroscopy time, and retrieval. The cohort consisted of 78 patients who had previously received Denali filters in successful placement. Seventy-one patients had both pre-procedural and pre-retrieval CTs for comparison. Thirty-five patients received filters via right femoral placement, 22 at the left femoral vein, and 14 at right jugular. Sixty-eight cases had attempted retrieval, all of which were successful. There was no significant difference in abutment or filter tilt among the three access sites. Abutment occurred in eight of the 71 patients with comparative CTs. Filter limb penetration occurred in two of the 71 patients. Other than one filter fracture that occurred during advanced retrieval, no other complications were observed. One variable that did result in a significant difference was length of fluoroscopy. Placement through the right jugular vein had an average fluoroscopy time of 117+/-37 s compared to right and left femoral placement (64+/-21 s and 67+/-15 s, respectively). All three access points were found to be similar in terms of associated complications, with a jugular approach being associated with longer fluoroscopy time [21]. Impact of access point was also studied by Grullon et al. who compared transjugular insertion with transfemoral. This study found increased risk of filter angulation (0.9% vs 0.34%; odds ratio [OR] 1.46 confidence interval [CI] 1.02-15 2.11; P=0.04) and rate of access site complications (0.25% vs 0.07%; OR 2.068; CI 1.01-4.23; P=0.048) in the transfemoral insertion group, with no significant difference in difficulty of retrieval between the groups [22]. It is important to note that although angulation rates and access site complications were higher in the transfemoral group, both insertion sites exhibited a low frequency of these complications. Both methods seem to be viable with specific patient anatomy and presentation playing a role in clinical decision making. Furthermore, trans popliteal insertion of IVC filters was found to be safe and efficient in a small retrospective study (n=21) performed by Kim et al. [23].
The safety of placement and retrieval of suprarenal IVC filters was analyzed by Baheti et al. In this retrospective study, 51 filters (40 Gunther Tulip, 10 Denali, and one Celect) were placed in the suprarenal position. Indications for this position included IVC thrombus, anatomical variants, and external IVC compression. Twenty-seven retrievals were attempted, all of which were successful; however, one retrieval was complicated with fracture struts. The median indwelling time was 87 days. No filter tilt or fracture was observed while the filter remained in place. There was also no significant change in renal function [24]. This study confirmed that if indicated, suprarenal positioning of IVC filters can be done safely and with low complications.

Outcomes: Filter-Related Complications
IVC filters are placed in order to prevent emboli from traveling to the pulmonary vascular bed. Complications with the filter may arise during placement, post-procedure, and during retrieval. Filter fracture and perforation are among the most common causes of failure, each with a positive correlation to longer indwelling time [25,26]. Indications for IVC filters vary widely within and across institutions. However, the types of filter-associated complications that arise are consistently seen across most studies.
Filter-related IVC perforation is defined by penetration of the IVC filter >3 mm into the wall of the vena cava, which accounts for about 20% of IVC filter complications per the MAUDE database. Retrievable filters have a higher rate of perforation, with most cases largely asymptomatic [27]. Asymptomatic perforations are often found incidentally on abdominal CT [28]. Similarly, there have been case reports in which patients were asymptomatic and perforation was noted incidentally during other intra-abdominal surgeries [29]. Perforations may present symptomatically within days to weeks. The primary complaint related to IVC perforations appears to be vague abdominal pain. Symptomatic cases accounted for one in 10 perforations and mostly required endovascular retrieval [30].
Fracture of filter limbs is directly related to the indwelling time of the filter. In very rare cases, the filter fragments can travel through the IVC and embolize in the heart or lungs. Fragments that travel to the right heart can present similar to pulmonary embolisms. Symptoms include shortness of breath, chest pain, and syncope. A case study by Thakur et al. from 2015 reported a linear structure moving with cardiac motion in the proximal right ventricle in a patient with chest pain. Fluoroscopy was used to visualize the IVC filter which showed normal position but visible fractures of at least two of the filter legs. The patient was made aware of the fractures and educated on possible risks regarding removal of the filter. The patient elected not to have the surgery performed and the fragment was left in situ [31]. The relative risk of fracture among different filters is currently a subject of study. Further investigation into fracture-prone filters will be able to assist clinicians in their medical decision-making.
Various single-center studies in different countries report rates of 10-20.6% of total filter complications with a large proportion attributed to thrombotic complications.
Shabib et al.'s report on a single Saudi Arabian facility over 11 years similarly found a complication rate of 20.6% in 382 filter insertions. Of these patients, recurrent DVT occurred at the highest rate (39%) followed by IVC thrombosis (32%), new/recurrent PE (18%) and other thrombosis (11%). Mechanical complications occurred in only 1.8% (seven patients), which consisted of filter tilting in six patients and IVC occlusion in one patient [32]. Nazzal et al.'s single institution study in Ohio consisted of 400 patients over a four-year period and found a 12.6% rate of complications, mostly due to thrombotic events; IVC thrombosis (4.75%), ipsilateral DVT (3.8%), PE (1.5%), filter migration/tile (1.5%) and hematoma at filter insertion site (1%) [33]. Wassef et al.'s study found 464 patients with IVC filters inserted over four years at a facility in Alberta, Canada. Overall IVC filter-associated complication rate was 22.2% (103). The most common complication was filter thrombosis (12.5%) and filter tilt (9.5%) [34]. Weinberg et al.'s study of a single Oklahoma (USA) center over three years reported 758 IVC filter placements. Insertion-related complications occurred in 4.1% of patients, accounted for by malpositioning and filter angulation. Filter-related complications within the first 32 days occurred in 19% of patients; 10.5% DVT, 4.2% PE, 3.8% IVC thrombosis [35]. A single-center study in Kyoto, Japan of 257 patients over eight years had a 10% rate of IVC filter-related complications that included 2.3% thrombus occlusion and single instances of infection and filter malpositioning [36].
In a study done by King et al. of 5780 IVC filter insertions across 62 US and Canadian facilities, they found a relatively low rate of IVC filter thrombosis (78 patients, or 1.3%) with two-year follow-up. However, based on their analysis of factors associated with filter thrombosis, they found an association with lack of antiplatelet therapy (hazard ratio 4.8, 95% CI 1.9-12.5, P=0.001) leading to their primary conclusion that antiplatelet therapy should be considered as a preventative measure against IVC filter thrombosis formation [37].
Hammond's report on 507 IVC filter placements in three UK centers over 12 years reports low complication rate associated with filter insertion (1.7%) despite an increasing trend of filter placement that mirrors US trends. There were two major complications (0.4%). One patient died in post-procedural recovery thought to be caused by PE. The second was IVC perforation within several hours of filter insertion, which required an emergency laparotomy. The remainder consisted of inadvertent placement, technical difficulty and wound oozing. Thirteen (2.6%) filtration-related complications occurred, including IVC occlusion (six patients, 1.2%), recurrent PE (four patients, 0.8%), infection (two patients, 0.4%), and filter migration (one patient, 0.2%). 24-hour and 30-day mortalities were 1 and 8%, and non-filter related [38].
Jung-Kyu et al. performed a retroactive observational study of 45 patient records from a single Korean institution over nine years for IVC filter-related complications based on appropriate follow-up CT imaging in patients with baseline presence of DVT and/or PE. The most common complication was IVC penetration (86.7%) and filter tip embedding suggestive of lateral tilting (51.1%). Filter thrombosis was suspected in 20% of patients. They found a 15-fold increased risk of significant IVC filter penetration if filters were left indwelling beyond 20 days (95% CI 3.6-68.7). However, there was no symptomatic complication in any of the reviewed charts [39].
Temporary IVC filters are the newest addition to the IVC filter market and as such, have the least amount of data regarding their utility or complications. One report came from Tokyo University Hospital (Miyahara) where two different temporary filters (Neuhaus Protect [Toray Medical, Tokyo, Japan] and Antheor [Boston Scientific]) were implanted in patients for various indications; 9.1% were contraindicated for anticoagulation, 12.1% for thrombolytic therapy, 84.8% perioperative prophylaxis, 3% DVT in pregnancy, and 15.2% prophylaxis without evidence of DVT. Though there was no incidence of PE-associated mortality, major complications arose in 27.3% of patients. These included 12.1% filter dislocation, fractured catheter in 9.1%, and catheter-related infection in one patient [40].  [42]. Interesting to this article is that the authors suggest actionable measures that could reduce the incidence of both immediate and delayed complications. Immediate complications (which include filter placement, misplacement, or insertion site-associated injury) can be avoided by early utilization of advanced imaging of the renal vein to ensure proper placement and deployment. Delayed complications (which include filter tilting/fracture/migration, thrombus formation/embolization, or vessel perforation) can be reduced given diligent follow-up and timely removal, implying proper follow-up protocols must be established at an institutional level. It is reasonable to argue that if these changes can demonstrably reduce complications, they should be implemented because patients receiving IVC filters are often already complicated or critical at the time of filter placement.
There are a set of complications that can occur during filter retrieval which are positively correlated with indwelling time. Filter fracture and IVC injury upon removal are possible outcomes [7,25,39,43,44]. These complications are outside the scope of our investigation and have been excluded in this article.

Outcomes: Mortality
Without randomized controlled trials to guide clinical decisions, a number of studies have relied on data from the National (Nationwide) Inpatient Sample (NIS) to evaluate IVC filter-related outcomes. The NIS is part of the Healthcare Cost & Utilization Project and contains information on over seven million hospital stays in the U.S. annually. The NIS is the largest publicly available healthcare database for informed decision-making on local, regional and national levels. NIS data is currently available for years 1988-2019. Since 2016, NIS data can be identified with ICD-10-CM/PCS diagnosis and procedural codes.
Dr. Paul Stein and colleagues have published several studies suggesting that certain populations of patients may derive mortality benefit from IVC filter implantation.
When analyzed by age group, Stein et al. (2016) found that IVC filters placed for primary diagnosis of PE without thrombolytic therapy modestly decreases mortality for patients of age >80 years old, even when accounting for comorbidities according to the Charlson Comorbidity Index [45]. Stein et al. (2017) did a follow-up study with data from the Premier Healthcare Database on patients >60 years hospitalized with PE and solid tumor malignancies. Among certain types of solid tumors, in-hospital all-cause mortality was lower in patients who had IVC filters (7.4%) vs. patients without (11.2%) (P<0.0001, relative risk 0.67). They also found a slightly lower three-month all-cause mortality with filters (15.1%) than without (17.4%) (P<0.0001, relative risk 0.86) [46]. Stein et al. (2019) investigated if IVC filter-related outcomes can be better seen when patients with PE are subdivided into stable or unstable categories, with unstable defined as in shock, or dependent on a ventilator. After stratification, unstable patients with an IVC filter have a greater reduction in mortality than those without (28.8% vs 46.3%, P<0.0001). Stable patients on the other hand had a lower, albeit not clinically meaningful, reduction in mortality rate than those without (5.8% vs 6.5%, P<0.0001) [47]. When controlling for immortal time bias,  found that regardless of thrombolytic therapy, unstable patients who received an IVC filter had a lower in-hospital all-cause mortality than those without (19.4% vs 40.8%, P<0.0001). Of note, the study mentioned that reduced mortality was associated with filters placed within one or two days of admission [48]. In both stable and unstable patients having undergone pulmonary embolectomy, Stein et al. (2020) found a reduction in mortality with an IVC filter; 4.1% vs. 27% (stable), 18% vs 50% (unstable), P<0.0001 for both sets. Here, mortality was improved only when filter insertion occurred within the first four to five days of admission [49].
Gul et al. studied IVC filter placement in PE complicated by pathologies like heart or respiratory failure, shock, and thrombolytic intervention. Patients with and without IVC filter placement with complicated PE were 1:1 propensity score matched for demographics, DVT, Elixhauser Comorbidity Index and other PE comorbidities. Their study found that IVC filter placement reduced mortality rates overall and in each subgroup, corroborating Stein et al.'s findings that IVC filters may provide greater benefit in gravely ill patients [50].
In recurrent PE, Stein et al. (2019) found that IVC filter insertion was associated with improved mortality in patients within three months of an index PE (3.0% with vs 39.3% without, P<0.0001). Specifically in the setting of recurrent PE without thrombolytic therapy or pulmonary embolectomy, stable patients may derive greater mortality benefit with an IVC filter than in other situations (2.6% with, 42.6% without, P<0.0001) [51].
Liang et al. looked specifically at short-term in-hospital mortality in patients with acute PE but found that the presence of IVC filter did not decrease mortality hazard for patients with acute PE than those without IVF filter (hazard ratio 0.93, 95% CI 0.89-1.01). Similar results were obtained for filter presence in high-risk patients with or without thrombolytic therapy (hazard ratio 0.85, 95% CI 0.6-1.21). Studies were done using propensity-weighted extended Cox analysis. Like Stein's 2018 and 2019 studies, Liang et al. accounted for immortal time bias and used similar NIS data, albeit with a slightly shorter time frame (Stein used years 2009-2014 whereas Liang's data was from 2009-2012). However, their conclusions differ as to whether IVC filters improve in-hospital mortality outcomes [52].
In a single hospital study of 248 patients, Jha et al. found that although an IVC filter was more likely placed when a patient had right heart strain or DVT (both P<0.001), there was no statistically significant difference between in-hospital mortality of patients with or without an IVC filter and acute PE (P=0.37) [53].
A systematic review and meta-analysis by Liu et al. elaborates on the limited potential benefit of IVC filters in patients with PE. The paper analyzed six studies from the USA, France and Australia assessing the use of IVC filters in 1274 adult patients with DVT and/or PE. Filter presence made no statistical difference in PErelated mortality (P=0.81) or overall mortality (P=0.13) within three months of filter placement or throughout the whole follow-up period up to eight years (PE-related mortality P=0.81, overall mortality P=0.61). They did, however, find that patients with IVC filters had an overall lower rate of PE occurrence than those without (3.2% vs 7.79%, 95% CI 0.25-0.71, P=0.001). Filter presence also reduced PE occurrence in patients at high risk of PE (P=0.01) and with absolute contraindication to coagulation (P=0.04). The placement of IVC filters had no significant effect on new incidence of DVT (P=0.58) [54]. While this paper and others question the survival benefit of IVC filters, Liu et al.'s findings suggest that filters may provide some protection against occurrence of PE in VTE patients.

Outcomes: Adjunctive Therapy
Though not an officially recommended usage, IVC filters are often used as adjunctive therapies to traditional anticoagulant or thrombolytic therapies which therefore warrants evaluation.
Isogai et al. investigated in-hospital mortality outcomes of 13,125 patients across 1015 acute care facilities in Japan who were hospitalized for PE, and upon admission, received antithrombotic or anticoagulant therapy with or without an adjunctive IVC filter. They found that patients who had a filter placed had a significantly lower in-hospital mortality rate (3948 patients, 2.6%) than those who did not (9177 patients, 4.7%, P<0.001, risk ratio 0.55, 95% CI 0.43-0.71). This finding was consistent after controlling for age, sex, pre-existing conditions, severity of disease, and therapeutic interventional procedures [55].
For patients with recurrent VTE within three months of starting anticoagulation therapy, Mellado et al.'s study suggests a more nuanced risk vs benefit of implementing IVC filters into treatment. This cohort study was assembled from the RIETE registry (Registro Informatizado de la Enfermedad Tromboemólica) and propensity score-matched groups were compared for survival benefit. They found a marked decrease in allcause death for patients whose VTE recurrence presented as PE (2.1% vs 25.3%, P=0.02). However, placement of IVC filters was not significant for reduction of mortality in patients whose VTE recurrence presented as DVT, nor for PE-related mortality [56].
Another cohort study by Muriel et al. identified a patient population (n=344) from RIETE, looking for IVC filter-related survival benefit in patients with acute symptomatic VTE and significant bleeding risk. Specifically, they analyzed clinical outcomes of mortality (all-cause and PE related) as well as recurrent VTE within 30 days of treatment between propensity-matched groups. After comparison of a 1:1 match of patients treated with and without IVC filters they found no significant difference in all-cause mortality, a slight decrease in risk for PE-related mortality with a filter (1.7% vs 4.9%, P=0.03), and increased VTE recurrence with presence of filter (6.1% vs 0.6%, P<0.001) [57].

Outcomes: Prophylaxis Before Catheter-Directed Thrombolysis (CDT) and Percutaneous Endovenous Intervention (PEVI)
IVC filters have been temporarily implanted in patients prior to catheter-directed thrombolysis procedures but several studies suggest that without IVC filters, there is a high risk for iatrogenic PE.
Kolbel et al. aimed to measure the frequency of filter embolization in 40 patients who underwent catheterdirected thrombolysis (CDT) for proximal DVT. After evaluating sequential phlebograms, visible emboli were found in 18 (45%) of patients ranging from <1 cm to >1 cm in size. Filters were removed after CDT and no patient developed symptomatic PE or significant filter-associated complications. Interestingly, patients with an underlying hypercoagulable disorder had fewer cases of IVC filter embolization than patients without (31% with, 69% without, 95% CI 0.02-0.56, P=0.006). Further analysis showed no significant differences in patient backgrounds or procedural factors [58]. Jiang et al. did a similar study with a slightly larger population of 189 patients undergoing CDT for acute proximal DVT, but in this group only eight of 189 patients (4.2%) were found to have IVC filter thrombus. No patient developed symptomatic PE following CDT and filter retrieval, procedural or thrombotic complication [59].
Akhtar looked at the adjunctive use of IVC filters in patients undergoing CDT for proximal lower extremity or caval DVT (NIS database, Jan 2005 -Dec 2013) and found no improvement in mortality on the basis of IVC filter placement (0.7% vs 1.0%, P=0.2). Moreover, Akhtar suggests that IVC filters may actually have increased burden on patients and hospitals given that filter placement was associated with higher rates of hematoma, in-hospital costs, and durations of admission [60].
Sharifi et al.'s study suggests that unlike CDT, prophylactic IVC filter usage may provide therapeutic benefit in percutaneous endovenous intervention (PEVI) for therapeutic removal of acute proximal DVT. In this study, they found that IVC filters reduced the incidence of iatrogenic PE (P=0.048). This was a single study of 141 patients, and warrants further investigating the role of IVC filters in conjunction to the procedure [61].

Comparison of Filter Types
Many filter types have been studied, varying from permanent to replaceable and bioconvertible. Peerreviewed articles from our searches suggest that IVC filter placement, particularly retrievable filters, can be performed safely, with better outcomes in terms of mortality and recurrence of PE, and with minimal complications. TrapEase (Cordis, Miami Lakes, FL, USA), Denali, Recovery (Becton Dickinson), Sentry bioconvertible, OptEase (Cordis), Gunther Tulip, Celect, VenaTech, Option, and Greenfield (Boston Scientific) filters are among the models that have been investigated.
Kalva et al. examined the safety and efficacy of the TrapEase IVC filter and found that 7.5% of patients developed PE following placement, with one death attributed to PE among the 751 patient cohort. Further findings included: filter fracture in 3.0%, thrombus trapped within the filter in 25.2%, thrombus extending beyond the filter in 1.5%, and near caval occlusion in 0.7%, with no case of filter migration [62]. This study concluded that the TrapEase vena cava filters are effective at preventing PE and can be placed with minimal complications. Tsui et al. also studied TrapEase filters, reporting breakthrough PE in 1.5% of patients. Recurrent DVT (18.7%) and filter fracture (13.3%) were among the other complications observed. Although the instance of filter fracture was moderately high, the authors note that there were no cases of free fracture fragments or distant migration [63]. Other filter models demonstrating similar reduction in PE include: Denali [64], Recovery [65], Sentry bioconvertible [9,10], OptEase [66,67], Gunther Tulip [68,69], Celect [70,71], Convertible VenaTech [11], and Option [72]. Greenfield filters were examined for safety and efficacy by Kazmers et al. in a single-center study. This study reported a major complication rate of 1.3%, with mean survival time following placement of 4.96 years (n=151), concluding that Greenfield filters could be placed safely with a low rate of misplacement [73]. Recurrence of PE, complications, and general findings of these studies are outlined in Table 1 [75] Not an endpoint n=255 (Celect) n=160 Perforation was seen in 49%, 43%, and 2% in Celect, GT, and Greenfield filters, respectively. Filter fracture occurred in 0.8%, 0.6%, and 0% in the Celect, GT, and Greenfield groups.
Greenfield filters had a significantly lower rate of perforation than Celect and GT filters. All three models had low incidences of fracture. Usoh et al. performed a randomized trial comparing Greenfield and TrapEase IVC filters, reporting 7% of cases with symptomatic IVC/IV thrombosis in the TrapEase cohort, and none in the Greenfield group (P=0.019). There were no instances of filter migration or perforation in either group. This study concluded that higher incidences of inferior vena cava thrombosis (IVCT) were likely attributable to the TrapEase filter and its structural characteristics promoting unfavorable hemodynamics [74]. Although the complications are characterized differently due to the comparison with Greenfield filters, the complication rates are largely in line with Kalva et al. in their research of the efficacy and safety of TrapEase filters [62]. McLoney et al. compared Greenfield, Gunther Tulip (GT), and Celect filters, noting higher perforation rates in GT and Celect (43% and 49%) than Greenfield (2%). In this study, all three models demonstrated a low frequency of filter fracture: 0.6% (GT), 0.8% (Celect), and no fractures in the Greenfield model [75]. A similar comparison was made by Koizumi et al. between GT, Trap/OptEase (TE/OE), ALN (Bormes-les-Mimosas, France), and VenaTech (VT) filters. This study found filter fracture in two cases with GT (0.7%), with one resulting in embolization to the pulmonary arteries. Filter fracture occurred with TE/OE filters in 14.1% of patients, with embolization in 2.2%. There were no incidences of filter fracture in the ALN (n=19) or VT (n=2) cohorts [76]. Kichang et al. compared retrievability and complication rates between Celect and Denali Infrarenal IVC filters at two-month indwelling. This randomized control trial (n=136) found a significantly higher rate of filter tilt >15° and strut penetration in the Celect filters (eight instances of filter tilt, 14 cases of strut penetration) versus the Denali (one instance of filter tilt, one of strut penetration) variety (P=0.033 and P=0.001, respectively). Three instances of breakthrough PE occurred in the Celect group with one occurrence in the Denali group [77]. Aggregate data seems to suggest that Greenfield filters exhibit a lower complication rate, with few instances of perforation, migration, and fracture. However, the variance in findings between these studies emphasizes difficulty in determining definitive guidelines and practices. There is agreement upon the reduction in the risk of PE, particularly in the patients with the most clear indications via the SIR guidelines [5], but complication rates vary between filters and studies.
Kai et al. compared outcomes for patients treated with permanent IVC filter versus temporary, and found recurrence of PE in none of the 25 cases using the temporary filter, but 18% in the 17 patients in whom a permanent filter was placed (P=0.10). Mortality rate was 35% in the permanent filter group and 16% in the retrievable group (P=0.14) [78]. Although this study is limited by its size, n=42, it does corroborate many of the aforementioned studies in terms of PE risk reduction. Likewise, Van Ha et al. retrospectively compared retrievable IVC filters (GT, Recovery) with permanent filters, reporting an average implantation time of 226 days for the retrievable models and 288 days for the permanent filters. Incidence of PE was similar in both cohorts, 1.4% in the retrievable and 1% in the permanent. The authors note an increasing rate of filter placement due to the possibility of retrieval and expanding indications, but conclude that in both permanent and retrievable models the risk of complications and recurrent PE are acceptably low [79].
Given the spectrum of filters available, our literature review indicates there is a role for IVC filters in thrombotic treatments, particularly in individuals at highest risk for recurrent PE who may be contraindicated to anticoagulants. The caveat seems to be the window of placement and removal. Many of the removable filter types showed a significant reduction in recurrent PE, however, as noted by Given et al., once beyond the ideal clinical window, filter complications increase and retrieval becomes more difficult [68].

Cancer-Related
Cancer patients are at a particularly high risk for developing thrombosis and subsequent embolism. In fact, venous thromboembolism is the second leading cause of death in cancer patients. Inherently, the risk rate for thrombus formation depends on the various types of cancer. There was an associated increase in risk of 180-day DVT [86]. However, this study is limited in that data on anticoagulation use was not provided, such that it is possible that more patients did have true indication for filter placement. Data also was not provided for cancer stage or filter retrieval status. Barginear et al. performed a retrospective study on 206 cancer patients with incidence of VTE to examine the efficacy of IVC filters compared to anticoagulation therapy. Overall survival was significantly greater in patients who received anticoagulation therapy (13 months) over the IVC filter group (two months).
Combining IVC filter with anticoagulation produced a median survival of 3.25 months (P<0.0002) [87]. However, a possible limitation to this study is that the patients in the IVC filter group showed contraindication to anticoagulation and tended to exist in the more advanced stages of cancer. This study did use multivariate analysis to adjust for this difference To examine the survival time of advanced cancer patients who received IVC filters, Mansour et al. performed a retrospective analysis of 107 cancer patients, the majority of whom were late stage. Filter insertion was without complications; however, DVT in 10 patients, PE in three, and one filter thrombosis. Specifically, regarding the advanced-stage patients, median survival time was 1.31 months in the 59 patients with available survival data. Thirty-nine percent of these patients expired within a month and 67.8% in less than three months [88]. Although IVC filters are a relatively safe option when there is high risk for bleeding and PE, the benefit to advanced-stage cancer patients is not very apparent. Mahmood et al. studied filter complications in patients with metastatic carcinoma vs localized carcinoma. Metastatic patients tended to have more filter-related complications (25% vs 11%, P=0.03) and decreased retrieval rates (31% vs 58%, P=0.01). An additional finding showed that reinitiation of anticoagulation therapy, if indicated, could reduce filter-related complications (OR 0.3; P=0.005) [89].

Surgery-Related
Bariatric surgery: Patients undergoing major surgery are at increased risk of developing DVT and may benefit from IVC filter placement prophylactically to prevent embolization to the lungs. Bariatric surgery patients carry a higher risk for VTE, but use of IVC filters in these cases remains controversial, as there are other risks associated with indwelling filters and difficulty of removal in obese patients [90]. Goldman [94].
In contrast, Giorgi et al. reported that in high-risk patients undergoing bariatric surgery 2.0% (n=49) had nonfatal DVT and PE, with no incidences of complications related to placement or removal [95]. Likewise, IVC filters were placed prophylactically in high-risk bariatric patients in a prospective study (n=107) performed by Sheu and colleagues. Postoperative DVT occurred in three patients within a three-month follow-up (3%, 95% CI 1-9%), with one instance of low-risk acute PE (1%, 95% CI 0.3%). No major filterrelated complications were reported in this study [90]. Schuster et al. found postoperative PE or DVT in 21% of patients (n=24) in a similar cohort [96]. Long-term outcomes in the bariatric surgery population were examined by Gargiulo, who reported DVT in 3.4% of cases (n=58) [97].
Given the contradictions between these studies, it would be difficult to make definitive statements about the use of IVC filters in the bariatric surgery population. It is noteworthy that the larger studies and metaanalysis seem to agree that IVC filter placement adds unnecessary risk of DVT, without any significant benefit in terms of reduction in PE.
Orthopedic surgery: Major orthopedic surgeries can also be an indication for IVC filter placement. Ahmed et al. examined the safety and efficacy of IVC filters in the prevention of PE for high-risk patients undergoing total hip or knee arthroplasty. Retrospective analysis showed a lower incidence of PE in the high-risk group for patients who received IVC filters (0.8% to 5.5%, P=0.028). There was no significant relationship between filter placement and postoperative VTE, DVT, and PE in the low-risk group [98]. Patients with DVT undergoing major orthopedic surgery may also be at high risk of PE. Huang et al. studied the prophylactic placement of IVC filters for the prevention of PE in patients undergoing surgery for spinal, pelvic, or lower extremity fractures. Groups were divided into patients with above-knee DVT (AKDVT), popliteal vein thrombosis (PVT), and below-knee DVT (BKDVT). There were no instances of symptomatic PE upon postoperative follow-up and significant differences in the occurrence of thrombosis within the filter between the groups (11.04%, 11.70%, and 8.06%, respectively) [99]. Although IVC filter insertion showed minimal benefit in the lower-risk groups, there does appear to be a benefit in surgical patients with larger risk factors.

Prophylactic Use and Trauma
Traditionally, IVC filters are placed in patients with absolute contraindication to anticoagulation therapy when there is risk for acute venous thromboembolism. However, indications in the clinical setting have controversially extended beyond this to include prophylactic use in patients at high risk for VTE, particularly in the case of trauma. Patients with multiple trauma or in a postoperative state are at high risk to develop a thrombus but severity of injury indicates immediate anticoagulant therapy to be harmful at this time. ). Thirty-one filters were removed with no complications within 25 days of placement after initiation of anticoagulant therapy. Forty-four filters remained; 41 were due to contraindication to anticoagulation and three due to thrombus caught within the filter. This study determined that temporary IVC filters can be safely placed by bedside ultrasound in critical patients until further anticoagulation therapy can be started [105]. Rosenthal et al. (2006) performed another study utilizing the Gunther-Tulip (n=49), Recovery (n=41), and OptEase (n=37) filters in patients with multiple trauma. There was no PE or other significant filter-related complications in 96.8% of patients. Three groin hematomas developed related to the filter. This study suggested that the Gunther-Tulip and Recovery filters could be placed for longer indwelling times if contraindication to anticoagulation persists. The OptEase filter, however, would require repositioning after 21 days. Ultimately, the Gunther-Tulip filter remained the easiest to retrieve with longer indwell times [106].
The efficacy of the Gunther-Tulip filter to prevent PE in interventional thrombolytic treatments was evaluated in a retrospective study by Yamagami et al. Fifty-five Gunther-Tulip filters were placed in 42 patients. There were no placement-related complications. One patient experienced perforation and filter migration. In-dwell time ranged from four to 37 days. Retrieval was attempted in 18 patients, with one failure. Twenty-four patients kept the filter as the DVT was resistant to alternative treatment. This study confirmed the safety and efficacy of the GT filter to prevent PE during DVT treatment. It was also found to be convenient when there is contraindication to removal [107].
The primary benefit of prophylactic IVC filters is to reduce risk of pulmonary embolism. A systematic review and meta-analysis by Haut  The device was placed safely at bedside without fluoroscopy. There was incidence of proximal DVT within the first seven days but no catheter-related infection. The primary goal of prevention of PE was achieved in 100% of the patients [117].
Another population at risk for DVT and subsequent PE is women during pregnancy. Between 1998 and 2004, Kawamata et al. examined 11 patients with DVT or who had developed DVT before pregnancy received an IVC filter. Anticoagulation therapy was started with filter placement but discontinued during intrapartum. There were no complications during filter insertion. There was no incidence of pulmonary embolism during the pregnancy period or after delivery, and all filters were removed, with one being replaced with a permanent filter [118]. For pregnant patients at risk for DVT and subsequent PE, IVC filters may be an effective intervention to reduce incidence of PE.
In recent years, rates of IVC filter placements have increased; however, follow-up and retrieval have not risen at a commensurate rate. Swami et al. performed a retrospective study to examine the indications and complications of IVC filters. Of the 254 cases examined, 65 were placed for absolute indication, 28 for relative, and 161 for prophylaxis. Complications appeared in 15 of the 96 cases with follow-up imaging. Only 19 filters were successfully retrieved [119]. This study suggested that prophylactic filters are being placed without strict follow-up for retrieval, which increases risk of filter-related complications such as filter migration or fracture.
As seen in these studies, filter retrieval is contraindicated when there is massive clot formation or complete occlusion of the IVC filter. Pan [120].

Long-Term Indwelling and Complications
IVC perforation and indwelling complications have been shown to increase over time. Wang et al. examined long-term complications in patients with an indwelling time of at least four years. This study found significantly higher IVC perforation and fracture rates in permanent filters compared to replaceable types. The authors concluded that IVC filter complications are relatively common with longer indwelling time. Moreover, it was reported that higher rates of fracture were found with CordEase and TrapEase filters, but IVC perforation rates were higher with retrievable conical-type devices [25]. Desai et al. retrospectively examined complication rates of long-term indwelling between retrievable and permanent IVC filters. Despite the fact that patients with retrievable indwelling filters were younger (mean age 62 vs 74, P<0.0001), this group had a significantly higher complication rate than those with permanent filters (9% vs 3%), P= <0.001) at mean 20-month follow-up. Complications of the retrievable and permanent IVC filters included thrombotic (4.4% and 2.2%, P=NS) and device-related (3% vs 0.5%; P<0.006) events. Even within matched groups following propensity score analysis revealed significantly higher complication rates in the retrievable filter group (9.1% vs 3.5%; P=.0035), suggesting that long-term indwelling of these models may be illadvised [121]. In concordance with these results, a recent retrospective study by Rauba et al. found a decreased rate of subsequent DVT (8.1% vs 11.9%; P=0.05; hazard ratio, 0.65; 95% CI, 0.42-1.00) and mortality (8.8% vs 28.8%; P<0.001; hazard ratio, 0.5; 95% Cl, 0.35-0.7) at mean follow-up time of 36±16 months for patients in whom the IVC filter was retrieved. Moreover, this study reported a negative association between longer indwelling time and likelihood of retrieval [122]. Of note, at mean follow-up 2.1 (0.68-4.78, n=112) years Ribas et al. found no cases of filter embolization, migration or fracture, but 12.5% had thrombotic complications including: filter thrombosis requiring long-term anticoagulation (4.5%), DVT (4.5%), and IVC thrombotic occlusion (3.6%). These included 57 patients with temporary filters and 55 with permanent filters [123]. Chow et al. confined their single-center observational study to include patients in whom permanent filters were placed, concluding that the permanent filters were effective at preventing recurrent PE but post-filter VTE and post-thrombotic syndrome were common, leading to a high morbidity rate. However, the authors did not attribute the morbidity to filter-related causes [124]. Iwamoto et al. found no incidences of recurrent PE, filter fracture, filter thrombus occlusion, or migration in 61 patients with permanent IVC filters and DVT during long-term follow-up (median: 18 months; mean: 28 months). There were several instances of PE and patient mortality within the first month, which was attributed to differences in underlying diseases and intracardiac thrombi [125]. Permanent IVC filters appear more appropriate for long-term indwelling and the authors suggest that, in combination with anticoagulation therapy, they can reduce the risk for fatal PE. In contrast, Lee (Jung-Kyu) et al. performed a retrospective observational study using CT imaging to determine major causes of IVC penetration and predictive factors for retrievable IVC filters. 87.6% incidence of IVC perforation (n=45) was reported, with associated longer indwelling time and diminished IVC diameter. Moreover, patients with indwelling time of >20 days were reported to have a 15.8 times greater risk of IVC penetration [39]. It would seem important to ascertain if the frequency of perforation would persist over a larger patient population, but each aforementioned study is in agreement about the dangers of lengthy in-dwelling times. Jaberi et al. sought out patients with retrievable filters that had still not been removed at median 3846.9 days since placement, concluding that, given the frequency of complications of long-term indwelling, filter removal should be performed if possible within the regulatory guidelines. This study concluded that IVC filter retrieval could be performed safely with low morbidity and mortality, reporting a success rate of 93% [43]. It is interesting to note that there seems to be general agreement regarding the dangers of long-term indwelling, however, when referencing regulatory guidelines there remains a need for consensus of best practice both in terms of indications and removal. Our review suggests that, even in longer indwelling periods, retrievable IVC filters can be removed safely.
IVC filter abutment against the IVC wall is one of the most common reasons for filter failure. Causes of abutment were investigated by Lee et al. who reported via multiple logistic regression model that a filter-tilt angle above 9.25° and external compression are independent risk factors (ORs: 4.56, 10.18, respectively). The authors conclude that CT imaging before IVC filter placement may help reduce the risk for filter tilt and external compression leading to fewer complications associated with abutment against the IVC wall [26]. Adding to this discussion, Laidlaw et al. found that larger filter diameter was associated with higher risk for filter tilting (P=0.0004). Moreover, greater filter tilt and prolonged dwelling time were more likely to require advanced retrieval techniques (P=0.01 and 0.002, respectively) [126]. Filter fracture frequency is also associated with longer indwelling times. Vijay et al. found median dwelling time for filter fractures of 692 days (range, 61-1,771 days), with no fractures found in filters in place for less than 61 days [127]. A prospective study performed by Ho et al. studied filter changes from prolonged indwelling. Filters from 100 patients were examined following removal at a median of 54 days in situ, reporting a positive correlation between duration in situ and loss of metallic elasticity of filter struts (Pearson correlation coefficient, 0.232; P=0.008) [128]. This change leads to a higher risk of filter fracture and reinforces the need for prompt removal.
Falatko et al. focused their study on patients >60 years old with IVC filters, and compared groups based on those who received anticoagulation therapy against those without. Mortality was the primary endpoint examined with 0.4 deaths per 1000 filter days and 0.7 deaths per 1000 filter days in the anticoagulated versus non-anticoagulated groups, respectively (P=0.06). The study concludes that the effect of anticoagulation was not significant in this cohort and notes age as a major confounding factor. Although anticoagulation is the treatment of choice for prevention of acute PE, in the elderly population there may be diminishing benefits [129].
Bistervels et al. studied women with indwelling IVC filters who become pregnant, reporting a complication rate of 5%. Complications were evaluated within six weeks postpartum and included filter migration, fracture, penetration, or filter thrombosis. Due to the low complication rate, the authors suggest that it can be safe to become pregnant with an indwelling IVC filter provided the filter is intact without signs of perforation. However, due to the few cases (n=20) the authors caution against making firm conclusions about the safety of IVC filters in pregnancy [130]. IVC filters can also be placed during pregnancy in patients with thrombotic risks. Konishi et al. retrospectively studied pregnant patients in Japan in whom a temporary IVC filter was placed due to risk of DVT. Among this patient group, there were two cases of filter complications with one related to an allergy to lidocaine while the other was a dislocation to the right atrium. Patients with IVC filter insertion experienced a later onset of DVT (22 vs 12 weeks; P=0.002) requiring a shorter duration of unfractionated heparin (16 vs 28 weeks; P<0.001), with no cases of PE occurring during the perinatal period [131]. The low reported complication rates suggest IVC filters may have utility in pregnant women who may have contraindications to anticoagulation or those in an acute window of risk for thromboembolic event.
Physiology can also play a role in filter complications. Laborda et al. examined IVC filters during Valsava maneuvers, reporting a 60% decrease in IVC cross-sectional area with an accompanying five-fold increase in pressure (P<0.001). Physical changes led to increased risks of filter strut fracture, abutment, and penetration [132]. These hemodynamic and mechanical pressures are more likely to occur if the IVC filter remains in situ for a prolonged period of time, thus increasing a patient's risk for filter-related complications. Although long-term indwelling exposes patients to further complications, these physiological variables could also be used to reduce the strain on the IVC filter. Hemodynamic changes and the inherent risks of patients requiring an IVC filter leaves them susceptible to infection. Prolonged indwelling increases this risk. Chua  Medicare claims data from years 2012-2016 shows a significant increase in the percentage of filter removal, but one must wonder why retrieval has not become the predominant outcome if the filters can be removed safely and the detriments of lengthy indwelling are apparent [12]. Non-retrieval of IVC filters has been suggested to be in part due to poor follow-up with patients [37]. Juluru et al. addressed the increasing numbers of retrievable, yet long-term indwelling, filters by proposing a filter-tracking software to reduce administrative burden while facilitating management of patients following IVC filter placement. The software would generate and track expected re-assessment dates, with options to update removal status, extension temporarily or permanently, and track survival status of patients. The software was tested at a single institution, and over the course of six months, tracked placement of 293 IVC filters and 83 retrievals. Compared to a control period of six months in the previous year, they found that with software facilitation, total retrieval rate was 34% compared to a control retrieval rate of 23%, though there was no significant difference in the retrieval time <210 days from placement (88.9 days control vs 102.7 days test; P=0.32). Of the filters retrieved >210 days after placement, filters were removed in the control group at a mean of 368.8 days, vs 242.5 days (P=0.03) [44]. A similar study was performed by Mikhael [134]. This suggests that implementing a longitudinal tracking method may increase the overall number of filter retrievals. Importantly, improved tracking and follow-up of patients with indwelling IVC filters could potentially provide insight on any potential associated long-term consequences for these patients.

Implications
Results from this systematic review suggest that there is a place for IVC filters in the reduction of recurrent PE for high-risk patients contraindicated to anticoagulation. There may also be a mortality benefit in these situations as well. This seems to be the most common clinical use with the broadest acceptance in practice.
A portion of the studies indicate a possible role for IVC filters for certain surgical patients. There does not appear to be broad agreement on the benefits in these cases and further studies are needed. Moreover, even in the cases when IVC filters are indicated and beneficial, it is important to note that the longer they remain indwelling, the more complications begin to accumulate. A majority of research concurs that there is positive correlation between indwelling time and complication rate, so it's important to keep in mind the relatively high rate of retrievable filters that are not retrieved [12,25,26,37,39,43,44,[121][122][123][124][125][126][127][128][129][130][131][132][133][134]. Finally, as has been mentioned in many studies regarding IVC filters, there remains a need for randomized control trials to determine their efficacy, particularly in regard to some of the expanding indications included in this review.
Articles reviewed in this paper are summarized and listed numerically and alphabetically (

Limitations
Our review was limited by the exclusion and search criteria outlined in the methods section. These criteria were used to narrow our focus to IVC filter placement and complications as they relate to thrombosis, however, there may be relevant information in research studies that did not meet the selection criteria. There may be complications related to filter placement and retrieval that are not thoroughly discussed. Additionally, because search parameters were limited to IVC filters and thrombosis or complications, there may be indications for use not included in this review. Moreover, our search was limited to three databases (PubMed, ScienceDirect, ProQuest), and may not include relevant papers accessible through other databases. Articles were excluded because full-text articles or versions in English were not available. This could result in relevant information being missed. Finally, there have been few randomized control trials performed to evaluate IVC filter efficacy and complications, making any conclusions preliminary with a need for further research.

Evidence Limitations
This review contains an abundance of information pertaining to IVC filters and their potential complications. However, it is not exhaustive. There are a limited number of randomized control trials related to filter placement. We have tried to highlight trends, using mostly retrospective data, but any conclusions are limited by the variability of experimental design. Some studies used mortality as a primary endpoint, while others used complications or recurrence of PE. Complications were measured during different timeframes. Most seem to agree that indwelling time is positively correlated with complications, however, measurements and timing of follow-up differ. This review is also inclusive of several articles written by Stein et al. These articles met our criteria for inclusion, but it is possible results have been more heavily influenced by these studies than by any other group of authors.

Conclusions
IVC filter placement and removal remains controversial, both in terms of indications for use and the safety and efficacy of treatment. While there are no consensus guidelines for best practice, there is compelling evidence that IVC filters can provide significant protection against PE with minimal complications if the treatment window is appropriate. Complications become more frequent with longer indwelling time, and despite risks of removal, evidence suggests that the benefits of retrieval outweigh the risks. There also seems to be a role for IVC filters in prophylaxis against PE. Research confirms reductions in PE for trauma patients within a bridging window and as perioperative care in coordination with anticoagulation. However, evidence demonstrates that the benefits of IVC filters tend to diminish in cases where anticoagulation is an option. Given the acute risks of PE, the use of IVC filters can be an important resource, particularly in patients with contraindication to anticoagulants. Subsequent removal is a beneficial step in the reduction of filter-related complications that would lower patient risk of mortality and morbidity. Finally, further research into the broader scope of indications may be necessary to elucidate the boundaries of when prophylactic filter placement outweighs risks. Moreover, agreement on treatment window and best practice guidelines would help improve clinical decision-making and reduce IVC filter-related complications.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.